I almost never see double grading in aikikai aikido. I feel like this has a lot to do with the complexity of technique, the way its taught, and the fact syllabi often start off at 6th/7th kyu. In my aikikai school I've never heard of someone double grading.

Double grading was not too uncommon in my karate organization, but we started at 10th kyu. By not uncommon, I mean like maybe one or two people a year would be double graded.

In my Yoshinkan school, I've never heard of someone double grading, and we have 10 kyu ranks. I have seen people promote faster than normal, given certain situations. At that school, techniques are taught very specifically, and there is a lot of memorization for weapons forms…all techniques are given a number grade and all of them have to be passed to move forward. So that standard is held regardless of frequency of testing.